Physics and Space Exploration

Pat McCracken, NASA Why does the shadow point at the moon

• Sky view from Ottawa, now
• lets turn off the sun, and track the planets now
• Stars appear to rotate in circles, centered on the North Celestial Pole.
• Stars near the equator set 4 minutes earlier each night.
• But we fix the stars in the sky
• The position of the sun varies during the year, being high in the summer and low in the winter. It traces out the ecliptic
• Most stars are relatively fixed: the moon and planets wander around the ecliptic.
• What we are actually seeing is a combination of 3 motions:
  1. The rotation of the earth on its axis. 1,2 and 3
  2. The rotation of the earth around the sun 4,5,6
  3. The orbit of the planets around the sun 7
  which can be very complicated!
• But if we could look at it from outside it is quite simple

Note that the effect of manned (or wommanned) exploration has been miniscule (the Moon is a very dull place!)

The Sun

A prominence erupts from the surface of the sun (7 light-minutes)

Mercury

The surface of mercury Similar to the moon but no mountains many scarps (probably formed by cooling of the planet) Very hot on sunward side (~500oC) Very cold on shadowed side (~-200 oC) No atmosphere

Degas crater

Orbital period of 88 days. Rotational period ∼ 56 days (Long thought to be ∼ 88 days: In fact, it is 2/3 of the orbital period).

Venus

Popular with writers: e.g C. S Lewis So does it look like this?

Almost featureless in optical. Usual picture is UV (upper) or infrared (lower) and only shows cloudtops. Venera, pioneer and radar showed surface for first time Year = 225 days. Rotation (i.e. 1 venus day = 243 days Retrograde (so sun "rises" in the west: unknown till 1961) Atmosphere very dense (pressure ∼ 100 × earth at surface). Mainly CO2.

Upper clouds rotate in 4 days (∼360 km hr-1) At surface, gentle winds, but temperature∼900 °C, Surface rocks basaltic, appear to be young

from Jim Imamura: http://zebu.uoregon.edu/~imamura/121/lecture-11/lecture-11.html

Radar maps show rough terrain as bright

Sapas Mons, a volcano 400 km across and 1.5 km high is on the western edge of Alto Regio. Note the lava flows extend for hundreds of km.

Mars

Very poular with writers: Bradbury did it best ("Sands of Mars") Lowell observed canals

Atmosphere: pressure ∼ 0.005 bar 95% CO2, rest O2, N2, Ar + very little H2O Temperature range 210 K->310 K

Two small, close, irregularly shaped moons. Phobos has very large impact crater.
Deimos Moons are probably captured asteroids.

VOLCANOES: Olympus mons: 25 km high, evidence of lava flows. Much larger than equivalent ones on earth (why?)
Candor Chasma Massive rift valley.

Many craters, at various stages of newness

The interesting problem: Does Mars have water? Sometimes it looks just as though it once did

This is the Newton crater

and what really look like arroyos in New Mexico

and "holes":deep caves where water could still exist. SO probably there was a lot of water, coiuld still be some underground

Hot off the press

Methane: tends to be created by living things (e.g. microbes ....)

NASA

Jupiter

Largest planet by far. Strongly banded appearance, corresponding to convective regions in atmosphere. Dark areas (bands) lie lower in atmosphere than light areas (zones).

Colours probably due to complex organic molecules: detected so far are: CH4 (methane), NH3 (ammonia), H2 (molecular hydrogen), C2H2 (acetylene), C2H5, PH3, H2O, G2H4 (germane), CO, HCN, H2S, top layer mainly ammonia.

Great Red Spot

: noted since 1600's ∼ 20,000 km × 50,000 km. Top of spot extends well above surrounding cloud tops. Note downstream eddies. Colour probably from organic molecules stirred up from below.

Speeds of rotation ∼ 500 km/hr Now clearly seen to be "hurricane" (lifetime not too surprising: 1000 × bigger than terrestial hurricanes, so lifetime could well be 1000 × longer!)

Cassini At Jupiter: Red Spot Movie Credit: CICLOPS, NASA, JPL, University of Arizona

Moons of Jupiter: Io

Jupiter has some of the oddest moons in the solar system. Four large easily visible with binoculars Can watch Io rotating
Io is in a state of continuous volcanic eruption: Volcanoes:Plumes to 250 km Vulcanism caused by "tidal pumping" by other moons.

Note surface is very unstable: no craters (age ∼ 106 yrs). "pimple" in centre is volcano seen frolm above

Europa:

Rock covered with ice, probably slushy since no impact craters.

Close-ups show odd crustal structures

Ganymede

Largest moon in the solar system: Ice on rock. Many craters, but with central pits, not peaks. Huge transverse faults

Saturn

Day 10 hr 14 mins. ATMOSPHERE similar to Jupiter, but less heating (internal &sun) so weather better

RINGS

First seen by Galileo as "Handles" Assumed to be solid, but Maxwell showed that tidal forces would have destroyed them... Spectrum consistent with small ice pellets and dust (moonlets).Voyager showed many thousands of ringlets, some rings elliptical F-ring very narrow & braided rings very thin (< 2 km) kept from dispersing by "shepherd" moons

Many Moons: Titan, Mimas, Tethys, Janus, and Enceladus.

Credit: Erich Karkoschka (University of Arizona Lunar & Planetary Lab) and NASA

• but the moons are weirdly different

• JANUS is two moons in orbit round each other.

Titan

Credits: ESA/NASA/JPL/University of Arizona

Touch down at 4.5 m/s, the saucer-shaped probe penetrated 15 cm. Surface consistency of wet sand or clay.

Titan Landscape Credit: ESA, NASA, Descent Imager/Spectral Radiometer Team (LPL)

Hyperion

Density about 1/2 water (!) suggests spongy texture!

Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

Iapetus

Half of moon is covered in material as black as coal

Enceladus

Giant striped snowball?

Uranus

Pale green. Uranus rotates on side. Note no bands, deep clear atmosphere.

Ariel from a distance of 170,000 kilometers.

Miranda from 42,000 kilometers.

Ring system, probably 9 narrow dark rings (seen by occulting star)

THis is how it might look from Ariel

Neptune

Pale blue-green Large dark spot

1 major moon, Triton has an atmosphere, and a retrograde orbit (captured asteroid?). Other smaller moons. Appearance similar to outer moons of Jupiter: i.e. ice-covered rock.

Pluto

Pluto Originally found in a search for 9th planet, based on prediction due to the perturbations of Neptune's orbit. Sometimes closer than Neptune (till 1999!)

> Seen to be double planet: Pluto-Charon

Asteroids

Paths of the earth (green), Mars (red square) Jupiter (yellow) twelve brightest asteroids

Most asteroid orbits lie in plane of solar system, a few are very tilted. Most lie between Mars and Jupiter, maybe more beyond Jupiter (Centaur asteroids). two Centaur Asteroids (outer Chiron and 1991DA). Orbits clearly imply that they were never part of a single object which exploded

Have now had close-up look at several asteroids. This is Gaspara

• Ida and its moon, Dactyl.

Eros

Eros is a lump of rock

We have managed to land on it

the fourth object (after the Moon, Mars and Venus) in the Universe! (but a bit too hard!)

Sedna

No easy answer: conventionally we take original 8 as planets, and say everything else is not (i.e. Pluto isn't).

Hubble

and above all, the Hubble which sees in the ultra-violet and infra-redand is above everything!

The Orbiting Hubble Space Telscope Credit: STS-103, STScI, ESA, NASA

Measurement of Distance

• Radius of Earth ∼ 6500 km
• Radius of Earth's orbit = distance to the sun ≈ 150 million km ≈ 1.5x10⁸ km ≈ 7 light-minutes

  i.e. it takes light 7 minutes to make the journey

• "Size" of solar system (not well defined) ≈ 2x10¹² km
• Popular one is the light year: distance traveled by light in 1 yr ~ 10¹⁶m.
• Astronomers usually use the "parsec": 1pc = 3*10¹⁶ m ∼ 4 lightyears.
• Closest star (α Centauri) is at a distance of ∼ 4 ly ∼ 1.3 pc.
• Sirius is at about 5 pc,

This shows a region in Andromeda: each star is about size of the sun. u And. first star found to have 3 planets. Note M31, M33 and a small cluster of stars: everything else are just Milky Way stars.

The best known cluster is the Pleiades: (Seven Sisters except we can only see 6 now)

A closer look: the Pleiades are a very young group of stars, about 107 years old, and very close: about 10 pc, so light takes 40 years to travel from them.

The milky-way is "our" galaxy: roughly 109 stars

• M31 is a massive spiral galaxy, rather like the Milky Way with about 1010 stars at 1.5 Mpc. About 20 kpc in radius

M33 is a slightly smaller galaxy, rather further away

M74 is another spiral

This is M87 (a giant elliptical galaxy) in Virgo. Almost perfectly spherical: about 1011 stars

A very pretty spiral (ESO 269), and note the very distant galaxies in the background. Stars are in our galaxy, at a distance of ~ 100 pc ESO 269 is at about 100 Mpc (i.e. 1023m Distant galaxies are at about 1 Gpc !

Galaxy Clusters

We have found about 108 galaxies. Galaxies form clusters: This is the VIrgo cluster: over 1000 galaxies: 3 big ellipticals, including M87 at the bottom. Closest big cluster

Galaxies Of The Virgo Cluster Credit & Copyright: Matt BenDaniel

This is the core of the Virgo cluster: M 84 and M 86 are the big ellipticals: also some small ellipticals and spirals

Credit & Copyright: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT

Coma cluster contains at least 104 galaxies

But this is only the beginning: We have measured the position of at least 10 million galaxies.......

and we can go deeper

And further: this is a cluster of galaxies

and further: this is a cluster of galaxies which is fairly close, but the most distant (for a week!) galaxy known is gravitationally lensed in the picture

• An hour to get to Neptune
• 4 years to get to the closest star
• 50000 years to travel across the galaxy
• 1.5 million years to get to the next galaxy
• But we can't travel at teh speed of light: we can manage 50 km/s!
• Can we beat the constraints somehow?
• Gravity assist
• Solar sails
• Plasma/ion drives
• Anti-gravity?
• Wormholes?

Gravity assist

• Collision of heavy object, vel v with stationary light object (Cannon ball hits ping-pong ball)
• ⇒ light object with vel 2v (!)
• Can't have direct collision but can arrange for close orbit ⇒ transfer of energy to probe
  
  done to push Ulysses probe out of plane of solar system

Radiation has pressure can harness with "solar sail" Upsides Free. Cumulative (since no friction Downsides Pressure very small and decreases with distance r from sun \color{red}{ P \sim \frac{1}{{r^2 }} } so .. Need huge, very light sail: Say 1 km2, 10 μ thick so Very vulnerable to meteors etc Only works in space (so still need rocket to escape earth) acceleration very small (maybe g/1000) But wouldn't a solar sailing ship be romantic Small ones should be launched soon

Plasma/ion drives

• e.g. 1 kg of gasoline has about E=10⁷J (10 Megajoules)
• Theoretical limit \color{red}{v \approx \sqrt E } (worse than this in practice, since we need to carry oxygen as well so this is about 3000 m/s for gasoline
• Can beat this with various exotic materials (e.g. hydrazine).
• However, can accelerate charged particles to (almost) speed of light c (well, 99.9% is easy).

• Very schematically

• Start with hydrogen
• Split into electrons and protons

• Put in electric field
• Accelerate to 99.9%c

• Downside

• Very inefficient: e.g ESA has just launched moon probe with ion drive that will take 15 months
• Still need rocket to escape earth's atmosphere

Space Elevator

A neat idea but needs materials with huge tensile strength and low density, and we still have energyy problems (doesn't matter how we get something into orbit, energy is a constant). Need to be at geostationary orbit ~ 36000 km e.g. NEED 150 ~1000 steel 5 GigaPascals 9000 kg/m3 Kevlar 2 GPa ~1000 Carbon nanotubes 150 GPa (in theory) ~1000

Wikipedia

Electromagnetic Rail Gun

Can accelerate metal object in varying mag. field (technology of (mag. lev. trains). Could build very long tunnel (50 km) to top of mountain, maybe speed of 3000 m/s at a height of 8000 m.,⇒ acceleration ~9g

• We know how to screen electric forces, because + and - charges attract each other, and can cancel out: however grav. forces just add and always attract. So can't manufacture cavorite.

  Wikipedia

• Might hope that, since matter attracts matter, it would repel anti-matter.
• Unfortunately anti-matter falls downwards (and theoretically has to)

Wormholes in space time?

• and there is a tooth-fairy
• but see later
• Space travel is one of the great themes. Now we'll look at time